1VFIO - "Virtual Function I/O"[1]
2-------------------------------------------------------------------------------
3Many modern system now provide DMA and interrupt remapping facilities
4to help ensure I/O devices behave within the boundaries they've been
5allotted. This includes x86 hardware with AMD-Vi and Intel VT-d,
6POWER systems with Partitionable Endpoints (PEs) and embedded PowerPC
7systems such as Freescale PAMU. The VFIO driver is an IOMMU/device
8agnostic framework for exposing direct device access to userspace, in
9a secure, IOMMU protected environment. In other words, this allows
10safe[2], non-privileged, userspace drivers.
11 12Why do we want that? Virtual machines often make use of direct device
13access ("device assignment") when configured for the highest possible
14I/O performance. From a device and host perspective, this simply
15turns the VM into a userspace driver, with the benefits of
16significantly reduced latency, higher bandwidth, and direct use of
17bare-metal device drivers[3].
18 19Some applications, particularly in the high performance computing
20field, also benefit from low-overhead, direct device access from
21userspace. Examples include network adapters (often non-TCP/IP based)
22and compute accelerators. Prior to VFIO, these drivers had to either
23go through the full development cycle to become proper upstream
24driver, be maintained out of tree, or make use of the UIO framework,
25which has no notion of IOMMU protection, limited interrupt support,
26and requires root privileges to access things like PCI configuration
27space.
28 29The VFIO driver framework intends to unify these, replacing both the
30KVM PCI specific device assignment code as well as provide a more
31secure, more featureful userspace driver environment than UIO.
32 33Groups, Devices, and IOMMUs
34-------------------------------------------------------------------------------
35 36Devices are the main target of any I/O driver. Devices typically
37create a programming interface made up of I/O access, interrupts,
38and DMA. Without going into the details of each of these, DMA is
39by far the most critical aspect for maintaining a secure environment
40as allowing a device read-write access to system memory imposes the
41greatest risk to the overall system integrity.
42 43To help mitigate this risk, many modern IOMMUs now incorporate
44isolation properties into what was, in many cases, an interface only
45meant for translation (ie. solving the addressing problems of devices
46with limited address spaces). With this, devices can now be isolated
47from each other and from arbitrary memory access, thus allowing
48things like secure direct assignment of devices into virtual machines.
49 50This isolation is not always at the granularity of a single device
51though. Even when an IOMMU is capable of this, properties of devices,
52interconnects, and IOMMU topologies can each reduce this isolation.
53For instance, an individual device may be part of a larger multi-
54function enclosure. While the IOMMU may be able to distinguish
55between devices within the enclosure, the enclosure may not require
56transactions between devices to reach the IOMMU. Examples of this
57could be anything from a multi-function PCI device with backdoors
58between functions to a non-PCI-ACS (Access Control Services) capable
59bridge allowing redirection without reaching the IOMMU. Topology
60can also play a factor in terms of hiding devices. A PCIe-to-PCI
61bridge masks the devices behind it, making transaction appear as if
62from the bridge itself. Obviously IOMMU design plays a major factor
63as well.
64 65Therefore, while for the most part an IOMMU may have device level
66granularity, any system is susceptible to reduced granularity. The
67IOMMU API therefore supports a notion of IOMMU groups. A group is
68a set of devices which is isolatable from all other devices in the
69system. Groups are therefore the unit of ownership used by VFIO.
70 71While the group is the minimum granularity that must be used to
72ensure secure user access, it's not necessarily the preferred
73granularity. In IOMMUs which make use of page tables, it may be
74possible to share a set of page tables between different groups,
75reducing the overhead both to the platform (reduced TLB thrashing,
76reduced duplicate page tables), and to the user (programming only
77a single set of translations). For this reason, VFIO makes use of
78a container class, which may hold one or more groups. A container
79is created by simply opening the /dev/vfio/vfio character device.
80 81On its own, the container provides little functionality, with all
82but a couple version and extension query interfaces locked away.
83The user needs to add a group into the container for the next level
84of functionality. To do this, the user first needs to identify the
85group associated with the desired device. This can be done using
86the sysfs links described in the example below. By unbinding the
87device from the host driver and binding it to a VFIO driver, a new
88VFIO group will appear for the group as /dev/vfio/$GROUP, where
89$GROUP is the IOMMU group number of which the device is a member.
90If the IOMMU group contains multiple devices, each will need to
91be bound to a VFIO driver before operations on the VFIO group
92are allowed (it's also sufficient to only unbind the device from
93host drivers if a VFIO driver is unavailable; this will make the
94group available, but not that particular device). TBD - interface
95for disabling driver probing/locking a device.
96 97Once the group is ready, it may be added to the container by opening
98the VFIO group character device (/dev/vfio/$GROUP) and using the
99VFIO_GROUP_SET_CONTAINER ioctl, passing the file descriptor of the
100previously opened container file. If desired and if the IOMMU driver
101supports sharing the IOMMU context between groups, multiple groups may
102be set to the same container. If a group fails to set to a container
103with existing groups, a new empty container will need to be used
104instead.
105 106With a group (or groups) attached to a container, the remaining
107ioctls become available, enabling access to the VFIO IOMMU interfaces.
108Additionally, it now becomes possible to get file descriptors for each
109device within a group using an ioctl on the VFIO group file descriptor.
110 111The VFIO device API includes ioctls for describing the device, the I/O
112regions and their read/write/mmap offsets on the device descriptor, as
113well as mechanisms for describing and registering interrupt
114notifications.
115 116VFIO Usage Example
117-------------------------------------------------------------------------------
118 119Assume user wants to access PCI device 0000:06:0d.0
120 121$ readlink /sys/bus/pci/devices/0000:06:0d.0/iommu_group
122../../../../kernel/iommu_groups/26
123 124This device is therefore in IOMMU group 26. This device is on the
125pci bus, therefore the user will make use of vfio-pci to manage the
126group:
127 128# modprobe vfio-pci
129 130Binding this device to the vfio-pci driver creates the VFIO group
131character devices for this group:
132 133$ lspci -n -s 0000:06:0d.0
13406:0d.0 0401: 1102:0002 (rev 08)
135# echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
136# echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
137 138Now we need to look at what other devices are in the group to free
139it for use by VFIO:
140 141$ ls -l /sys/bus/pci/devices/0000:06:0d.0/iommu_group/devices
142total 0
143lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:00:1e.0 ->
144 ../../../../devices/pci0000:00/0000:00:1e.0
145lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.0 ->
146 ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.0
147lrwxrwxrwx. 1 root root 0 Apr 23 16:13 0000:06:0d.1 ->
148 ../../../../devices/pci0000:00/0000:00:1e.0/0000:06:0d.1
149 150This device is behind a PCIe-to-PCI bridge[4], therefore we also
151need to add device 0000:06:0d.1 to the group following the same
152procedure as above. Device 0000:00:1e.0 is a bridge that does
153not currently have a host driver, therefore it's not required to
154bind this device to the vfio-pci driver (vfio-pci does not currently
155support PCI bridges).
156 157The final step is to provide the user with access to the group if
158unprivileged operation is desired (note that /dev/vfio/vfio provides
159no capabilities on its own and is therefore expected to be set to
160mode 0666 by the system).
161 162# chown user:user /dev/vfio/26
163 164The user now has full access to all the devices and the iommu for this
165group and can access them as follows:
166 167 int container, group, device, i;
168 struct vfio_group_status group_status =
169 { .argsz = sizeof(group_status) };
170 struct vfio_iommu_x86_info iommu_info = { .argsz = sizeof(iommu_info) };
171 struct vfio_iommu_x86_dma_map dma_map = { .argsz = sizeof(dma_map) };
172 struct vfio_device_info device_info = { .argsz = sizeof(device_info) };
173 174 /* Create a new container */
175 container = open("/dev/vfio/vfio, O_RDWR);
176 177 if (ioctl(container, VFIO_GET_API_VERSION) != VFIO_API_VERSION)
178 /* Unknown API version */
179 180 if (!ioctl(container, VFIO_CHECK_EXTENSION, VFIO_X86_IOMMU))
181 /* Doesn't support the IOMMU driver we want. */
182 183 /* Open the group */
184 group = open("/dev/vfio/26", O_RDWR);
185 186 /* Test the group is viable and available */
187 ioctl(group, VFIO_GROUP_GET_STATUS, &group_status);
188 189 if (!(group_status.flags & VFIO_GROUP_FLAGS_VIABLE))
190 /* Group is not viable (ie, not all devices bound for vfio) */
191 192 /* Add the group to the container */
193 ioctl(group, VFIO_GROUP_SET_CONTAINER, &container);
194 195 /* Enable the IOMMU model we want */
196 ioctl(container, VFIO_SET_IOMMU, VFIO_X86_IOMMU)
197 198 /* Get addition IOMMU info */
199 ioctl(container, VFIO_IOMMU_GET_INFO, &iommu_info);
200 201 /* Allocate some space and setup a DMA mapping */
202 dma_map.vaddr = mmap(0, 1024 * 1024, PROT_READ | PROT_WRITE,
203 MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
204 dma_map.size = 1024 * 1024;
205 dma_map.iova = 0; /* 1MB starting at 0x0 from device view */
206 dma_map.flags = VFIO_DMA_MAP_FLAG_READ | VFIO_DMA_MAP_FLAG_WRITE;
207 208 ioctl(container, VFIO_IOMMU_MAP_DMA, &dma_map);
209 210 /* Get a file descriptor for the device */
211 device = ioctl(group, VFIO_GROUP_GET_DEVICE_FD, "0000:06:0d.0");
212 213 /* Test and setup the device */
214 ioctl(device, VFIO_DEVICE_GET_INFO, &device_info);
215 216 for (i = 0; i < device_info.num_regions; i++) {
217 struct vfio_region_info reg = { .argsz = sizeof(reg) };
218 219 reg.index = i;
220 221 ioctl(device, VFIO_DEVICE_GET_REGION_INFO, &reg);
222 223 /* Setup mappings... read/write offsets, mmaps
224 * For PCI devices, config space is a region */
225 }
226 227 for (i = 0; i < device_info.num_irqs; i++) {
228 struct vfio_irq_info irq = { .argsz = sizeof(irq) };
229 230 irq.index = i;
231 232 ioctl(device, VFIO_DEVICE_GET_IRQ_INFO, &reg);
233 234 /* Setup IRQs... eventfds, VFIO_DEVICE_SET_IRQS */
235 }
236 237 /* Gratuitous device reset and go... */
238 ioctl(device, VFIO_DEVICE_RESET);
239 240VFIO User API
241-------------------------------------------------------------------------------
242 243Please see include/linux/vfio.h for complete API documentation.
244 245VFIO bus driver API
246-------------------------------------------------------------------------------
247 248VFIO bus drivers, such as vfio-pci make use of only a few interfaces
249into VFIO core. When devices are bound and unbound to the driver,
250the driver should call vfio_add_group_dev() and vfio_del_group_dev()
251respectively:
252 253extern int vfio_add_group_dev(struct iommu_group *iommu_group,
254 struct device *dev,
255 const struct vfio_device_ops *ops,
256 void *device_data);
257 258extern void *vfio_del_group_dev(struct device *dev);
259 260vfio_add_group_dev() indicates to the core to begin tracking the
261specified iommu_group and register the specified dev as owned by
262a VFIO bus driver. The driver provides an ops structure for callbacks
263similar to a file operations structure:
264 265struct vfio_device_ops {
266 int (*open)(void *device_data);
267 void (*release)(void *device_data);
268 ssize_t (*read)(void *device_data, char __user *buf,
269 size_t count, loff_t *ppos);
270 ssize_t (*write)(void *device_data, const char __user *buf,
271 size_t size, loff_t *ppos);
272 long (*ioctl)(void *device_data, unsigned int cmd,
273 unsigned long arg);
274 int (*mmap)(void *device_data, struct vm_area_struct *vma);
275};
276 277Each function is passed the device_data that was originally registered
278in the vfio_add_group_dev() call above. This allows the bus driver
279an easy place to store its opaque, private data. The open/release
280callbacks are issued when a new file descriptor is created for a
281device (via VFIO_GROUP_GET_DEVICE_FD). The ioctl interface provides
282a direct pass through for VFIO_DEVICE_* ioctls. The read/write/mmap
283interfaces implement the device region access defined by the device's
284own VFIO_DEVICE_GET_REGION_INFO ioctl.
285 286-------------------------------------------------------------------------------
287 288[1] VFIO was originally an acronym for "Virtual Function I/O" in its
289initial implementation by Tom Lyon while as Cisco. We've since
290outgrown the acronym, but it's catchy.
291 292[2] "safe" also depends upon a device being "well behaved". It's
293possible for multi-function devices to have backdoors between
294functions and even for single function devices to have alternative
295access to things like PCI config space through MMIO registers. To
296guard against the former we can include additional precautions in the
297IOMMU driver to group multi-function PCI devices together
298(iommu=group_mf). The latter we can't prevent, but the IOMMU should
299still provide isolation. For PCI, SR-IOV Virtual Functions are the
300best indicator of "well behaved", as these are designed for
301virtualization usage models.
302 303[3] As always there are trade-offs to virtual machine device
304assignment that are beyond the scope of VFIO. It's expected that
305future IOMMU technologies will reduce some, but maybe not all, of
306these trade-offs.
307 308[4] In this case the device is below a PCI bridge, so transactions
309from either function of the device are indistinguishable to the iommu:
310 311-[0000:00]-+-1e.0-[06]--+-0d.0
312 \-0d.1
313 31400:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev 90)
315